The study of children with Williams syndrome, a neurodevelopmental disorder with a known genetic basis, is fascinating for those interested in understanding the genetic underpinnings of behavior. The unusual peaks and valleys in cognitive abilities, coupled with increasingly specific delineation of the genes deleted in individuals with this disorder, have led to discoveries of likely relations between specific genes and cognitive abilities. Evidence is mounting that children with Williams syndrome also have distinctive personality characteristics, including a strong interest in interacting with other people. In light of this sociability, some researchers have pointed to Williams syndrome as a stark contrast to autism spectrum disorders, where reciprocal social interaction is an area of specific weakness. However, given that children with Williams syndrome have been shown to be sociable but not necessarily socially successful, the nature of their abilities warrant further investigation. The overall objective of this investigation is to provide a more complete description of the socio-communicative behavior of young children with Williams syndrome, so as to gain a more comprehensive sense of the characteristics that are distinctive to the disorder. To achieve this objective, the proposed investigation will examine the behavior of children with Williams syndrome in a standardized context, using the Autism Diagnostic Observation Schedule. Participants will be 2 groups of 35 2 1/2- to 5 1/2- year-olds, matched for chronological age, developmental level, and gender. One group will consist of children with Williams syndrome; the other group will consist of children with other developmental disabilities. The specific aims are to: (1) Describe the socio-communication abilities of young children with Williams syndrome and determine the distinctiveness of their pattern of abilities. (2) Determine the relation between chronological age, intellectual functioning, and the presence of deficits in communication and reciprocal social interaction. (3) Explore the extent to which the behavior seen in young children with Williams syndrome overlaps with the diagnostic category of Autism Spectrum Disorders and the nature of this overlap. The results of this investigation should lay a foundation for increased understanding of relations between genes, personality, and social behavior and facilitate the design of more effective social skills interventions for children with Williams syndrome or other developmental disabilities.